The transpsoas approach provides a relatively easy and safe surgical corridor to the anterior and lateral aspect of the vertebral column and disk space without the need for retraction of the peritoneum or mobilization of the great vessels. The lateral corridor provided allows the spine surgeon to place a relatively large graft more anteriorly, thereby facilitating the restoration of disk height and lumbar lordosis while proving good axial support. Indirect foraminal decompression and unbuckling of posterior longitudinal ligament can be achieved through restoration of disk height. Additional stabilization can be provided using lateral plate or percutaneous posterior instrumentation (Fig. 29.2). A recent biomechanical study by Fogel et al. [8] found that the laterally inserted cage alone can provide significant reduced range of motion in flexion-extension; additional stabilization with lateral plate, posterior spinous process plate, or bilateral pedicle screws can provide additional stability in lateral bending and axial rotation, but the exact type of fixation used did not make a statistically significant difference in stability. The original indication for lateral interbody fusion delineated by Ozgur et al. was for patients with low back pain associated with degenerative disk disease but without severe central canal stenosis. Subsequently, the transpsoas approach was increasingly performed for degenerative disk disease or adjacent segment disease where interbody fusion is desired, most commonly from L1 to L5 [9–12]. With the increased familiarity with the transpsoas approach and advancement in spinal instrumentation technology such as expandable cages, the application of the lateral approach has been broadened to include treatment of vertebral fractures, tumors, and spinal deformities [7]. Transpsoas approach has also been used for lumbar disk replacement with favorable results [13]. Table 29.1 summarizes the indications suitable for the transpsoas approach. Relative contraindications to this technique may include vascular abnormalities precluding access, significant spondylolisthesis, previous retroperitoneal surgery, and severely collapsed disk spaces.

The anatomical structures relevant to the transpsoas approach include the external and internal oblique muscles, transversus abdominis muscle, transversalis fascia, retroperitoneal fat, quadratus lumborum, psoas muscle, and the lumbar plexus [12]. A thorough understanding of local anatomy is essential for optimal surgical outcome and complication avoidance. The external oblique, internal oblique, and transversalis muscles are three layers of muscle that form the lateral abdominal wall. The transversalis fascia is immediately deep to the transversus abdominis muscle, covering the retroperitoneal fat. The retroperitoneal fat has a characteristic yellow appearance that serves as a helpful indicator of the presence of retroperitoneal space. In the retroperitoneal space, the quadratus lumborum muscle originates from the last rib and the transverse processes of the upper lumbar vertebrae and inserts to the internal lip of the iliac crest. The psoas muscle is situated anterior to the quadratus lumborum; it originates from the transverse processes and lateral aspect of the lumbar vertebrae and joins the iliacus muscle inferiorly and insert into the lesser trochanter of the femur. The transverse process and quadratus lumborum muscle are good anatomical landmarks for the posterior boarder of the psoas muscle.

The lumbar plexus is formed by the lumbar nerve roots with minor contribution from the T12 root (Fig. 29.3a). It travels between superficial and deep parts of the psoas major muscle. Major branches of the lumbar plexus include the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, obturator, and femoral nerves in addition to the lumbosacral trunk. The superior part of the plexus is usually located in the posterior one-fourth of the L1 to L2 vertebral body and travels progressively more anteriorly as the lumbar plexus descends. With the exception of the genitofemoral nerve, majority of the lumbar plexus branches are located in the posterior half of the L1 to L4 vertebral body, which makes the anterior half of the L2 to L4 vertebrae the optimal surgical corridor for the transpsoas approach [11]. Uribe et al. [12] and Moro et al. [11] in a cadaveric study nicely demonstrated the lumbar plexus anatomy in relation to the transpsoas approach and divided the area between the anterior and posterior edges of the vertebral body into four zones, Zone I (anterior quarter), Zone II (middle anterior quarter), Zone III (middle posterior quarter), and Zone IV (posterior quarter), as shown in Fig. 29.3b. The safe zone to prevent direct nerve injury from L1–L2 to L3–L4 was located at the middle posterior quarter of the VB (midpoint of Zone III), and the safe anatomical zone at the L4–L5 disk space was at the midpoint of the VB (Zone II–Zone III demarcation). The genitofemoral nerve is formed by the L1 and L2 nerve roots and assumes a more anterior course (Zone II at the L2–L3 space and in Zone I at the lower lumbar levels L3–L4 and L4–L5) than the rest of the lumbar plexus, which predisposes itself for injury during transpsoas approach especially at L3 and below which may result in pain and paresthesias in the medial thigh and scrotal area. Apart from within the psoas muscle, there is potential risk of injury to the ilioinguinal, iliohypogastric, and lateral femoral cutaneous nerves in the retroperitoneal space where these nerves run along the posterior abdominal wall and then course obliquely, inferiorly, and anteriorly across the surgical corridor within the abdominal muscles to reach the iliac crest and the abdominal wall and it should be kept in mind [14]. Transpsoas approach to the L4–L5 disk space may be associated with higher risk of injury to the lumbar plexus and can often be limited by a high-riding iliac crest. Flexing the operating table or resection of the iliac crest can help to obtain lateral access in the setting of high-riding iliac crest. Transpsoas approach to the L1–L2 disk space may be limited by a low-lying 12th rib, which may require rib resection or intercostal approach. Though attempted in cadaveric studies with additional surgical maneuvers, L5–S1 is typically not amenable to transpsoas approach without significant neurological complications due to the traversing lumbar plexus at this level [15].